1. Field of the Invention
The present invention relates to a method of optically recording/reproducing data on/from a recording medium, a reproducing apparatus to which the method is applied, and a recording medium applied to the method and reproducing apparatus.
2. Description of the Related Art
In a conventional recording medium, the pitch of tracks on which data is recorded is determined on the basis of a minimum diameter (W) of a spot of a reproducing laser beam, which spot is formed when the beam is converged on the recording medium. The beam spot diameter (W) is given by EQU W=1.22.times..lambda./NA
where .lambda. is the wavelength of a recording/reproducing laser beam, and NA is the numerical aperture of an objective lens for converging the laser beam.
Under general conditions, .lambda.=0.8 .mu.m and NA=about 0.5, and accordingly W=2 .mu.m.
The beam spot of such a laser beam has a Gaussian distribution of intensity. If the beam spot, which is directed to a target track, is displaced to an adjacent track, the data recorded on the adjacent track is superimposed on a reproduced output and a reproduction error may occur (cross-talk effect).
Thus, the track pitch is determined so as not to cause the cross-talk effect.
In general, the track pitch (TP) is set to be equal to the diameter of that component of the beam, whose intensity at the center of the intensity distribution is 1/e.sup.2. When the above numerical values are applied, TP=1.6 .mu.m. The track comprises a land and a beam guide groove for enabling the converged beam to scan along the center line of the track.
It has recently been proposed that the track pitch (TP) be reduced to increase the recording density of the recording medium. When the track pitch is decreased, the cross talk increases. As a result, the beam diameter needs to be reduced. For this purpose, for example, the wavelength (.lambda.) of the laser beam is reduced, or the NA of the objective lens is increased. However, since the wavelength (.lambda.) of a beam emitted from a conventional semiconductor laser or laser diode (LD) is limited to a range near the infrared, there is a limit for reduction of the beam diameter. In addition, since focus servo operation becomes difficult because of the influence of aberration due to non-uniformity in the thickness of the recording medium and a decrease of beam waist, there is a limit for an increase in the NA of the objective lens.
In order to solve the above problems and enhance the recording density, the following have been proposed.
As a first example, Published Examined Japanese Patent Application (PUJPA) No. 57-138065 discloses a method wherein the difference in height between adjacent tracks is set to be equal to .lambda./8 to .lambda./4 of the wavelength of a reproducing beam, whereby a main signal is not influenced by pit data recorded on the adjacent track. A push-pull method is adopted in the tracking servo for this optical system; thus, the crosstalk component is decreased and the influence due to the crosstalk effect reduced.
As a second example, PUJPA No. 58-155528 discloses a method wherein a reproducing beam is irradiated onto a track with a v-cross section, and different pit data are recorded on the two inclined surfaces of the track. Reflected beams from the two inclined surfaces are received by different light-receiving elements, thereby reproducing data. Thus, the crosstalk at the time of reproduction can be reduced.
According to a third example, three beams as reproducing beams are scanned on the groove and land on which data is recorded. Specifically, data on a desired groove (or land) is reproduced by a center beam, and data on the lands (or grooves) on both sides of the desired groove (or land) is reproduced by side beams preceding or following the center beam. According to this method, the output from the side lands (or grooves) is canceled from the output from the center groove (or land) by an electronic circuit, thus eliminating crosstalk components.
PUJPA No. 59-207433 shows a fourth example wherein a reproducing beam is scanned on a groove and a land on which relative height data is recorded in a synchronized state, and data on the groove and land is simultaneously reproduced by a push-pull method.
As a fifth example, there is known a method of increasing a data transfer rate. A typical method is to increase the rotation speed of the recording medium, but this method has a problem in precision of servo. To solve this problem, a method has been proposed wherein signals are reproduced in parallel by using a multi-head or a multi-beam. According to this method with an array of four laser diodes (LD), simultaneous reproduction is effected by four beams.
In the first example, data of only one track is reproduced at a time, and the data transfer rate at the time of reproduction is increased.
In the second example, it is technically difficult to form v-tracks precisely and uniformly. In addition, complex control is required to enable a laser beam to trace the v-track.
In the third example, since side beams for detecting crosstalk components precede or follow the center beam, a time lag occurs in canceling the crosstalk components.
In the fourth example, although a high recording density and a high transfer rate can be attained, the recording method is based on phase pit formation, and therefore this method is applicable only to a read-only memory.
In the fifth example, the mechanism for tracking and focusing is complex, and high precision is required for positioning between the track and laser beam.